At an event billed as a “campaign kickoff,” Lehigh County Executive Tom Muller announced he will not seek a second term in office Monday — to the surprise of many who were gathered to hear him speak.

Muller’s supporters had come to Brookside Country Club in Lower Macungie Township in response to an invitation that strongly implied that Muller, a Republican turned Democrat, was seeking re-election. A header at the top of the invitation included Muller’s campaign logo, and donations were requested for the Government That Works political action committee.

But 15 minutes into his speech, Muller delivered news that caught many off guard.

“As great as this chapter of my life has been and continues to be, it will come to an end this year,” Muller said to the silent crowd. “I know that will come as a surprise to most of you. This is not the kickoff of my campaign tonight.”

“Oh?” said a woman in the back of the room, confused.

Instead, Muller used the opportunity to introduce his handpicked successor: Democrat Phillips Armstrong. Armstrong, a Whitehall Township commissioner and recent candidate for the 183rd State House District, was not at Monday’s announcement. He was instead attending a Whitehall commissioners meeting.

PHOTO GALLERY: Lehigh County Executive Tom Muller spoke at an event Monday night at Brookside Country Club where he announced he would not be running for re-election.

(ADDISON GEORGE / SPECIAL TO THE MORNING CALL)

“I couldn’t close out this chapter in my life without knowing that there would be someone on the ticket who could continue running the county for the citizens and not for political ideology with the whims of a couple wealthy contributors,” Muller said. “I do have that person, and that’s Phil Armstrong.”

After a momentary pause, Muller’s supporters applauded.

When Muller first ran for office in 2013, some speculated that the first-time office holder would be a single-term executive. Muller came to the job after a stint as county administrator, and he worked many years in the private sector. But as the Feb. 14 date to begin petitioning for signatures to secure a spot on the ballot drew closer, it appeared more likely that Muller was running.

Muller said Monday that he and his wife plan to move away from the Lehigh Valley in several years after their daughter graduates from high school. It wouldn’t be fair to serve only part of a four-year term, he said.

Armstrong, 67, is a retired social studies teacher and basketball coach who worked at Whitehall High School for 40 years. He has served on the Whitehall Township Board of Commissioners since 2014, and is currently the board president.

Armstrong ran last year for a state House seat vacated by Rep. Julie Harhart. He lost to Zach Mako, Harhart’s handpicked successor.

Armstrong is the second candidate to enter the race for Lehigh County executive. Brad Osborne, a Republican second-term county commissioner, kicked off his campaign for the office last month.